1. Field of the Invention
This invention relates to semiconductor fabrication and more particularly to an improved method for forming a field oxidation isolation structure by implanting nitrogen into the active regions.
2. Description of the Relevant Art
The fabrication of an integrated circuit involves placing numerous devices in a single semiconductor substrate. Select devices are interconnected by a conductor which extends over a dielectric which separates or "isolates" those devices. Implementing an electrical path across a monolithic integrated circuit thereby involves selectively connecting isolated devices. When fabricating integrated circuits it must therefore be possible to isolate devices built into the substrate from one another. From this perspective, isolation technology is one of the critical aspects of fabricating a functional integrated circuit.
A popular isolation technology used for an MOS integrated circuit involves the process of locally oxidizing silicon. Local oxidation of silicon, or LOCOS process involves oxidizing field regions between devices. The oxide grown in field regions are termed field oxide, wherein field oxide is grown during the initial stages of integrated circuit fabrication, before source and drain implants are placed in device areas or active areas. By growing a thick field oxide in field regions pre-implanted with a channel-stop dopant, LOCOS processing serves to prevent the establishment of parasitic channels in the field regions.
A conventional LOCOS process is shown in FIGS. 1-6. In FIG. 1, a thin oxide 12 is grown on a semiconductor substrate 10. A layer of silicon nitride, shown in FIG. 2 as layer 14, is then deposited on the thin oxide 12 with a chemical vapor deposition process. Photoresist is then deposited on the silicon nitride layer 14 and patterned to obtain a patterned photoresist layer 15 shown in FIG. 2. Photoresist layer 15 is patterned to define an active region 16 and a field region 18 in semiconductor substrate 10.
After patterning photoresist layer 15, silicon nitride layer 14 and thin oxide layer 12 are removed in regions where the photoresist has been removed. FIG. 3 shows the wafer after the exposed silicon nitride layer 14 and thin oxide layer 12 have been removed and photoresist layer 15 has been stripped. In FIG. 4, the wafer is inserted into a thermal oxidation tube to grow an oxide in field region 18 of semiconductor substrate 10. This thermal oxidation is represented in FIG. 4 as number 20. As is well known, the presence of silicon nitride layer 14 over active region 16 suppresses the growth of oxide in those regions. FIG. 5 shows a partial cross section of the wafer after completion of thermal oxidation 20. A thick field oxide 22 is present in field region 18 of semiconductor substrate 10 while a thin oxide 24 has been formed underneath silicon nitride layer 14 in active regions 16. A transition region between thick field oxide 22 and thin oxide 24 comprises the well known bird's beak 26. Bird's beak 26 extends into active region 16 by the amount d.sub.1. Completion of the conventional field oxidation LOCOS process is accomplished by stripping the silicon nitride layer 14, removing thin oxide 24 with a wet etch process, growing a sacrificial oxide layer, and removing the sacrificial layer. The sacrificial oxide process is performed to eliminate the so called "Kooi" ribbons of silicon nitride that can form at the interface between thin oxide 24 and semiconductor substrate 10 during the field oxidation process. The sacrificial oxide layer is shown in phantom as a dashed line in FIG. 6.
While LOCOS has remained a popular isolation technology, there are several problems inherent with the conventional LOCOS process. First, a growing field oxide extends laterally as a bird's-beak structure. In many instances, the bird's-beak structure 26 can unacceptably encroach into the device active area 16.
In addition, it is well known that depositing CVD silicon nitride directly on a silicon surface results in a very high tensile stress that can exceed the critical stress for dislocation generation in silicon. This stress, which can create fabrication induced defects in the silicon substrate is believed to be caused by the termination of intrinsic stresses at the edges of the nitride film. To prevent this stress from affecting the semiconductor substrate 10, it is necessary in the LOCOS process to form the thin oxide layer 12, commonly referred to as a buffer oxide layer, thereby adding complexity and expense to the process.
Still further, the removal of silicon nitride layer 14 after formation of field oxide 22 is known by those skilled in the art of semiconductor processing to generate a large number of particles. When silicon nitride layer 14 is subjected to the high temperatures used to grow field oxide 22, a thin film forms on an upper surface of silicon nitride layer 14. The thin film is believed to comprise a composite of silicon, oxygen, and nitrogen (Si.sub.x O.sub.y N.sub.z). Removal of this film from the upper surface of silicon nitride layer 14 requires additional processing steps. Instead of simply immersing the wafer in a hot phosphoric acid solution, as is commonly done to wet etch silicon nitride layers, a brief plasma etch must be performed to remove the Si.sub.x O.sub.y N.sub.z film from the upper surface of silicon nitride layer 14. In addition to undesirably removing oxide from an upper surface of field oxide 22, this plasma etch process is known to generate a large number of particles. These particles can become lodged on the silicon surface that can result in defective devices. Finally, the conventional LOCOS process requires a sacrificial oxide to remove the Kooi ribbons as described above. The formation and removal of the sacrificial oxide layer adds additional processing time and expense to the conventional LOCOS process. In addition, the sacrificial oxide is generally over-etched to ensure that all traces of silicon nitride formed during the field oxidation process are removed. The over-etch undesirably reduces the thickness of the field oxide 22.
Therefore, it would be highly desirable to implement a field oxidation process sequence which eliminated the need to deposit a silicon nitride layer on the wafer surface. By eliminating the nitride deposition step, such a process would thereby eliminate the high particle counts associated with the nitride strip process and would eliminate the field oxide etch back cycle required in conventional field oxidation process sequences. Further, the elimination of the nitride deposition from the field oxidation process would eliminate the need to grow the Kooi oxide after removal of the nitride layer. In addition, the bird's beak structure and the encroachment problems associated with conventional field oxidation processes would be substantially reduced.